This invention relates generally to apparatus and method for testing AC motor power systems and more particularly apparatus and method for nondestructive in-circuit auto self testing of an AC motor system including inverter power circuit, its drive circuits and the load motor.
It is common in AC electric motor drive systems to employ power inverter systems to furnish electric power from a DC source to the AC motor. These inverters are of various types, but are often comprised of gate turn-off thyristors (GTO) in a bridge arrangement. The GTOs in the bridge are selectively gated to control the electrical power supplied to the motor by converting the DC power into AC power which drives the motor. Typically, two GTOs are connected in a series arrangement in what is commonly referred to as a "leg" between the two sides of the DC source. A common converter of this type is a three-phase converter having three legs connected in mutual parallel between the positive and negative DC source lines. The GTOs of each of the legs are rendered conductive in a predetermined order or sequence in order to control the electrical power delivered from the DC busses to the motor load.
These motor system require require regular maintenance to avoid or detect a variety of common failure modes. For example, if both GTOs of a leg were to become conductive simultaneously, there would exist a short between two of the DC source lines which, if allowed to continue, could result in great damage to the load, power source, and/or to the GTOs. In addition, failure of various components and segments of the system such as the feedback circuits, drive circuits, and the motor load itself can occur. In prior art, expensive and time consuming manual off-line testing has been necessary to detect many errors at an early enough stage to ensure proper operation. In order to enhance maintenance of such systems, it would accordingly be highly desirable to provide an auto self test capability. However, in the prior art, self testing has been largely limited to testing the failure of the GTOs and detection of certain abnormalities in drive port wave form.
It is accordingly an object of the present invention to provide a novel method and apparatus for auto self testing of the AC motor system which provides for testing and detection of a wide variety of abnormalities and failures.
It is another object of the invention to provide a novel method and apparatus for auto self testing that provides nondestructive incircuit testing of an AC motor drive system including testing the motor coils without the risk of rotating the motor during the test.
It is yet another object of the invention to provide a novel method and apparatus for auto self testing of an AC motor drive system which utilizes the motor load inductance and circuit snubber capacitance to permit comprehensive testing.
Briefly, the foregoing and other objects of the invention are achieved in accordance with the present invention by testing a power converter and motor system wherein the power converter has a plurality of legs having first and second series connected control turn-off devices, such as GTOs, each having a control electrode. The testing method includes selectively generating control signals and coupling them to the control electrode of each control turn-off device to render each device conductive, one at a time, such that a transient current flows through the motor and the respective conductive control turn-off device. The transient current is measured and then compared to a reference to detect defects in the motor and power converter.
The invention may further provide for testing the generation of the control signals by detecting the state of the control signals being coupled to the control electrode and verifying that they are identical to the control signal generated. In addition, the state of the control turn-off devices may be monitored and then compared to the control signals to detect failure of a control turn-off device to respond to the control signal. A voltage measurement may also be made in each leg of the power converter a selected time period after transient current is measured and the voltage compared to a reference to detect additional defects. This test process may be controlled by a programmed processor to provide a programmed auto self test of the motor system.